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Diagenetic Fluid and System of Dolomite Reservoir in Huanglong Formation, Eastern Sichuan Basin, China

Wen, Huaguo *1; Zheng, Rongcai 1; Dang, Lurui 2; Zheng, Chao 2; Chen, Haoru 1
(1) State Key Laboratory of Oil and Gas Reservior Geology and Exploitation,CDUT, Chengdu, China.
(2) Chongqing Gas District of Southwest Oil and Gas Company of CNPC, Chongqing, China.

The dolomite of the Carboniferous Huanglong Formation (equivalent to Moscovian) is one of the most significant natural gas reservoirs in Eastern Sichuan Basin. Based on the comprehensive study of the texture-genetic classification of dolomite, the diagenetic behavior and evolution, the trace (e.g. Fe, Mn and Sr) and rare earth elements (REE), as well as the stable isotopes (e.g. C, O and Sr), from the combined characteristics of basin-scale diagenesis, four subsystems of different diagenetic stages are identified in Huanglong Formation according to the “hydrological system”, including pore water, compaction brine, meteoric water and thermobaric water. Three types of diagenetic fluids are closely related to the development of the dolomite reservoir in each subsystem: (1) the burial formation brine conserved in the early Sabkha had lower Sr, strong Eu negative anomalies, slightly negative δ13C and δ18O and high 87Sr/86Sr ratio. As the burial depth increased, the reducibility and temperature of the fluid increased gradually.
(2) The strong-oxidizing and low-temperature meteoric water in open paleo-epidiagenetic stage had the Fe and Mn enrichment, slightly high negative Ce anomalies, negative δ13C and δ18O and extremely high 87Sr/86Sr ratio.
(3) The strong-reducing and deep-seated mixed thermal brine in reburial diagenetic stage had Mn enrichment, Fe depletion, weakly positive Eu anomalies and obviously negative δ13C and δ18O. The origin and nature of the fluid in each diagenetic system had the inherited familiarity of development and evolution. And the mechanism of water-rock reaction, the diagenetic behavior, the products and their combined characteristics were various. Consequently, we offer some new ideas of the controlling factors and distribution for the dolomite reservoir development: (1) Facies belts of the barrier beach and offshore shoal controlled the regional distribution and the size of the reservoir; (2) Early diagenetic burial dolomitization in formation brine subsystem was the basis for the reservoir development; (3) The meteoric water karstification in paleo- epidiagenetic stage expanded the distribution and the size of the reservoir, and improved the reservoir qualities; (4) The corrosion in deep subsurface and the tectonic fracturing in reburial diagenetic subsystem of thermobaric water further improved the porosity and permeability of the reservoir.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California